Mini but mighty

They might be tiny but carbon nanotubes are stronger than steel. Now chemists are finding ways to put this strength to use on a more everyday scale, as Helen Dell finds out

Carbon nanotubes have fantastic properties, says Martin Cadek, development manager at SGL Carbon in Bonn, Germany. They have very high electrical conductivity, they are the best thermal conductors ever discovered (better even than diamond), and they are incredibly strong. Their stiffness (Young’s modulus), for instance, is 19 times that of steel, once the two materials’ different densities are accounted for.

Cadek and his colleagues aim to borrow some of these phenomenal properties to put into materials for use on a larger, every-day scale.

’The idea is that if we mix carbon nanotubes into a polymer the resulting composite will have a combination of the properties of both,’ says the group’s principal investigator Jonathan Coleman, at the Materials Ireland Polymer Research Centre, Trinity College Dublin. The polymer is easily manipulated and shaped, and the nanotubes should add a reinforcing element - much the same way as steel is used to reinforce concrete.

The group started by just mixing nanotubes with the polymer before it solidified, hoping the polymer would stick to the nanotubes. But they found that, although this simple approach worked for a few polymers, in most cases there is very little interaction between the polymer and the nanotubes. So they began to look for a more general approach that could be used with any polymers.

’What we thought was, if we could attach any given polymer strand to the nanotubes, then when we mix those nanotubes with a solvent, the attached polymer strands would tend to make the nanotubes soluble, and if we added the same polymer to the solution, then there would be a compatibilitising effect,’ explains Coleman. Essentially the polymer should form strong contacts with its own strands, meshing the nanotubes into the bulk polymer.

So they turned to chemist Yurii Gun’ko, also at Trinity College Dublin, to search out a chemical route to covalently attach polymer strands directly to the nanotubes.

Gun’ko knew that C₆₀ (fullerene) can react with alkyl lithium and Grignard reagents to produce alkylated metal fullerides, and he decided to apply a similar scheme to nanotubes. He took advantage of slight imperfections in the nanotubes, where they deviate from the purely hexagonal ’graphitic’ structure - the tetragonal, pentagonal and heptagonal rings that occur at the tips or in bends in the nanotubes, for example. ’All the imperfections are extremely reactive,’ he points out.

The team used butyllithium to attack the defects. Then they reacted the lithiated nanotube with chlorinated polypropylene, producing an exchange reaction that eliminates lithium chloride. This resulted in the covalent bonding of the polypropylene to the nanotube surface by C-C bond formation.

’The covalent bonding of the polymer to the nanotubes results in very high interfacial stress transfer,’ says Coleman. This transfer is the single most important point in terms of reinforcing any material with a filler - essentially they have to stick well together.

Using the technique to incorporate just one weight per cent of nanotubes in the polypropylene increased the strength of the material by three- to four-fold (depending on the measure of strength used). The reinforced polypropylene is almost as strong as steel, according to Cadek.

’The beauty of this is that the chemical reaction that we used to attach the polypropylene to the nanotubes is a generic route, so this can be applied to any polymer,’ says Coleman. Moreover, according to Gun’ko, the process should scale up easily to industrial levels, and the nanotubes, although fairly expensive, are fast becoming affordable (they cost about € 30 per gram now, compared with € 180 six months ago). ’The technology will be relatively cheap and feasible to be applied in the near future,’ he says.

The group are now working on improving the technique further, by attaching alkanes to the nanotubes. ’Alkanes, being hydrophobic, should be able to mix with lots of polymers, eg polystyrene,’ explains Coleman. The alkylated nanotubes should thus be able to strengthen many different polymers without needing nanotube-binding reactions specialised to each specific polymer. The group’s preliminary results suggest that this is indeed the case.

’We seem to see that to get the best bonds, you need longer compatibilising [alkane] molecules,’ Coleman told Chemistry World. He suggests that the longer chain length also allows the nanotubes to become more soluble. This will be important because in many cases there is a limit to the amount of nanotubes that can be mixed into the polymer before they start aggregating and become insoluble. ’If you increase the solubility you can increase the nanotube loading level,’ he says. So the properties of the nanotube-polymer matrix will be doubly improved: by increasing the strength of the bond between the polymer and the nanotube filler, and by increasing the amount of filler that can be used.

The results are ’very exciting’, says Ray Baughman, director of the NanoTech Institute, University of Texas, Dallas, US. ’I think there are all sorts of exciting possibilities for nanotube enhanced structures.’

The new polymers will have applications in the aerospace industry, amongst others, according to Cadek. They are lightweight and strong, and the nanotubes’ electrical conductivity allows static charges that build up during flying to be safely dispersed. ’At the moment the industry uses paints to disperse these, but these paints are heavy and it’s an additional working step,’ he says.

And Coleman’s group is already working on ways of using nanotubes to make electrical and thermal insulators: if heat or charge can be dispersed rapidly enough in the plane of a material, it will be prevented from crossing such an insulating layer, they suggest.

Such multifunctional applications, using the strength of nanotubes combined with their other properties, interest Baughman. His group, for instance, have made carbon fibres strengthened with nanotubes that can act as supercapacitors to store electrical energy. The carbon nanotube fibres are almost as strong as DuPont’s fibrous, tough material, Kevlar, but they are much tougher, in that they can absorb much more energy - Kevlar’s toughness is around 30J/g, whereas carbon nanofibres can absorb over 550J/g before breaking. ’They are very tough and they can be used to absorb energy, eg from bullets, but also provide energy storing capabilities for soldiers,’ Baughman suggests.

So it seems as though these minute structures will soon be having a large impact. ’I believe that organometallic chemistry of nanotubes will have a great impact on nanotechnology in the near future,’ concludes Gun’ko.

Helen Dell is a freelance science writer based in London, UK